Electrically heating continuously traveling metal strip



May 11, 1948. F. MIESS I ELECTRIGALLY HEATING CONTINUOUSL Y TRAVELING METAL STRIP Filed March 50, 1944 \k m& Q

Q I\ mvcwzgig 9 O o 0 O O O 0 O o Ni Q fi/Ji iidr/iqy Patented May 11, 1948 ELECTRICALLY HEATING CONT INUOUSLY TRAVELING METAL STRIP Fred Miess, Gary, Ind.

Application March 30, 1944, Serial No. 528,747

4 Claims. (Cl. 21911) This invention relates to metal heating broadly but is particularly concerned with continuously heating metal strip. One usual way of heating metal strip is to cause the strip to longitudinally travel continuously through a continuous furnace provided with metal heat radiators in the form of either metal tubes inside which a combustible mixture burns or electric resistance elements. At the present time the metallurgical art cannot provide commercially usable metal, for making these heat radiators, which can safely operate above about 2000 F., so it is impossible to heat the metal to higher temperatures although this would be desirable in some instances. Although higher temperatures might be obtained by burning flames directly inside the furnace this is undesirable because it damages the metal being heated. Since presently available furnace refractory materials can be operated safely up to temperatures of 2400 F. and higher, the temperature limiting factor is set by the metal from which the heat radiators must be made.

With the above in mind, the object of the present inventor is to provide for heating metal, particularly continuously traveling metal strip, to higher temperatures than the metal heat radiators now available can be operated; namely, to temperatures above 2000 F. and, in fact, to temperatures limited only by available furnace refractory materials.

A specific example of apparatus embodying the features of the invention is schematically illustrated by the accompanying drawings, in which:

Figure 1 is a vertical longitudinal section; and

Figure 2 is a cross section from the line II-II in Figure 1.

More specifically, these drawingsshow metal strip S being continuously fed from a coil l upwardly and around a roller 2 and, from this roller, horizontally to pinch rollers 3 from which the strip vertically drops downwardly through a heating furnace 4, having metal heat radiators 5 and generally constructed according to prior art principles, to guiding rollers 6 inside this heating furnace and close to its exit. Assuming the heat radiators 5 are of good construction it is possible to raise the temperature of the strip in this heating furnace to or almost to 2000 F., this assuming the furnace to be of adequate length or the strip traveling speed to be adequately slow. However, the temperature of the strip cannot be raised to higher temperatures because of the limiting factor imposed by presently available metal from which the heat radiators 5 can be made. In the case of the present invention it is not necessary to raise the temperature of the strip this high in this heating furnace, it being considered preferable to raise the temperature of the strip only so high as will assure its not being marked by the rollers 6 and which will permit proper operation of these rollers.

Upon leaving the rollers 6, the strip continues to drop and immediately passes through a second continuous strip heating furnace I also having metal heat radiators 8 and whose operation ordinarily would be limited by the fact that they must be made from presently available metal. However, this continuous heating furnace I differs from the furnace 4 by having a passage 9, made of thermal insulation, through which the strip travels and which completely separates the strip circumferentially from the metal heat radiators 8. This passage 9 may be made of refractory furnace material, as can the other furnace wall parts, and functions to shield these heat radiators from heat radiation from the metal strip S when its temperature goes above the temperature at which these metal heat radiators can safely operate, which the strip temperature does for reasons which will subsequently become apparent.

The strip leaves the exit of the furnace I through a cooling chamber ill from which the atmosphere is continuously removed by way of a conduit ll, passed through a cooler I2 and returned to the cooling chamber by way of a conduit [3, with a blower I4 providing the motive power, the strip then encountering guiding rollers [5. The strip is in the form of an unsupported span between the guiding rollers =6 and the guiding rollers l5 and these rollers function to keep the strip centered as it travels through the passage 9 and the cooling chamber 10.

From the rollers I5 the strip turns and travels through equipment presently to be described, opportunity being now taken to explain how the strip can reach the higher temperatures mentioned as it travels through the thermal insulating passage 8. This is done by putting electric current into the strip as it enters the furnace 4 and leaves the as yet undescribed equipment, this being accomplished by the use of electrical rollercontactors l6 connected by lines l6a to an electric generator ll, so that the electric current passes through the strip length between these roller-contactors IS in series with all of its longitudinal portions. The generator ll provides suflicient electric power to produce efiective electric resistance heating of the traveling metal strip carrying the current, but the amount of current 3 required does not exceed that which would. normally heat the strip but slightly.

As the strip passes through the furnace 4 its temperature is just beginning to rise and its electrical conductivity is relatively good, but as the strip continues through this furnace 4 and its temperature continues to rise its electric resistivity increases so that the heating efiect provided by the electric resistance heating increases, it being obvious that the electric resistance heating effect bears a relation to the electrical resistivity of the metal strip. As the strip enters the thermal insulating passage 9 of the furnace 1, its temperature continues to rise'because of the electric resistance heating it is receiving in conjunction with there being no heat radiation loss and, perhaps, because the heat radiators 8 at the entrance end of the furnace 1 have heated that end of the passage 9 sufficiently so that its inside surfaces are hotter than the strip, However, the electric resistance heating soon raises the temperature of the strip to and beyond the safe operating temperature of the metal heat radiators 8 of the furnace land the full effect of the present invention then comes into play. In explanation, the thermal insulating passage 9 then begins to shield the metal heat radiators 8 from heat radiation from the metal strip so that, in conjunction with judicious powering of these radiators 8, it is possible to keep these radiators at or below their safe maximum operatingtemperature. At the same time, the electric resistance heating effect need only be sufiicient to boost the temperature of the strip above that to which it Was heated in the furnace 4. The-maxi mum temperature to which the strip can 'be heated by the combined effects described is the maximum safe operating temperature of presently available furnace refractory materials from which the passage 9 must be made, but since this is considerably higher than the safe -maximum operating temperature of presently available metal heat radiators, the invention is ef fective in attaining the object of the inventory prevents marking of the strip by the rollers l5.

and permits proper operation of these rollers.

The operation of the present invention is complex enough to warrant some further explanation. Electric current is continuously passing in series through all portions of the strip and the cool strip is first acting as a relatively good electrical conductor so that little electric resistance heating of the strip is obtained. As the strip enters the furnace 4 its temperature begins to rise because of the heat radiated to it by the metal heat radiators 5, and at the same time the electric resistance heating efiect increases because the resistance of the strip increases with its temperature. The strip cannot be heated in the furnace 4 to a temperature exceeding the safe operating temperature of the metal heat radiators, namely about 2000 F.,but it canbe raised to this temperature and this will be aided by the electric resistance heating which has increased with the temperatureand electrical resistivity of the strip so that it is an effective iactor. Electric resistance heating of strip is normally commercially impracticable because of the large amount of current required, but it is made commercially practicable by the just described practice sincelittle or no radiation loss is involved because of the heat exchange between the strip and the metal heat radiators 5. However, if the electric resistance heating power is sufliciently high, and suflicient power is provided the metal heat radiators 5, the strip temperature will rise so high that the strip will be marked by the rollers 6 and the latter will not operate effectively, and, What is of greater importance, the strip, after exceeding the maximum safe operating temperature of the metal heat radiators 5, will radiate more heat to these heaters than it receives from them and even though this may be but a hundred degrees or so it will resultin themetal heat radiators 5 operating at higher temperatures than they can safely work orwhich is commercially practicabl because of the very short heater life that will result. Keeping the above in mind, the strip is preferablypreheated in the furnace 4 so that its temperature is only raised to the maximum permitting proper-working of the guiding rollers 6, or, if these guiding rollers B are eliminated so that their operational difliculties need not be considered, the temperature of the strip still should not be raised above the safe operating temperature of the metal heat radiators 5. Therefore, assuming the strip enters the passage 9 at safely below the maximum temperature indicated, for instance at about 1800 or 1900 F., the heat radiators 8 in the furnace 1 should be controlled adjacent this end of the furnace sothat they are operating close to but safely below their maximum safe operating temperature, or about 2000 F. At this time, the strip may be receiving more heat from the inside of the passage 9 than it is radiating to this inside, but its temperature is rapidly increasing due to the electric resistance heating which is rendered eifective due to the lack of radiation loss.- Furthermore, at this time, namely as the strip is just beginning its travel through the passage 9, the outsides of the walls of this passage may be at higher temperatures than their insides because the passage is made of thermal insulation. Veryshortlythe strip temperature rises to the maximum safe operating temperature of the metal heat radiators 8 and what may well be termed equilibrium is attained, In other words, the metal strip has a temperature of about 2000 F., the heat radiators 8 at this zone are operating at about 2000 F. and there is -no temperature gradient through the walls of the passage 9. It is to be understood that the radiators in all-instances should be arranged for individual or zone control along the furnace length, which is conventional practice in conventional continuous heating furnaces. From this point on in the travel of the strip through the passage 9, the strip temperature rises above the safe operating temperature of the metal heat radiators .and power on the latter must be reduced or adjusted to keep them at their safe maximum operating temperatures. This temperature increase of the strip is the result of the electric resistance heating caused by the passage of current through it, this creating heat inside the strip at a rate now approaching a maximum since the temperature of the strip is approaching its maximum. To exemplify, the strip may now have a temperature of 2200" F. and will be radiating heat to the inside of the passage 9 so that this insidehas a temperature of about 2200 F; but the radiators 8 in this zone must be con trolled to maintain .thematf2000 F., this mean ing'thatthere-is a 200 F. temperature drop through the -thermalinsulating passage between its inside and outside, this passage now functioning to shield the metal heat radiators 8 to prevent them rising above the 2000 F., which effect cannot be obtained without the thermal insulating shielding. Along with this effect, electric resistance heating of the strip is made practical since it need only raise the temperature of the strip 200 F. to boost the strip temperature above the 2000 F. that can be provided by radiation from the metal radiators and to which the outside of the passage may be thereby heated to reduce heat loss from the strip. The described effect proceeds to occur with increasing degree, and the strip temperature is limited only by what furnace refractories can withstand, as has been previously noted. In addition to all this, the electric resistance heating effect is concentrated where it is needed by the fact that the electrical resistance of the strip increases with its temperature, the relatively cool portions of the strip functioning mainly as electrical conductors carrying current to the point where the electric resistance heating is needed to boost the temperature of the strip above that to which it can be driven by putting heat into the strip from metal heat radiators.

The strip is shown traveling horizontally from the guiding rollers l5 over carrying rollers 18 through a reheating furnace l9 similar to the furnace 4, this furnace also having metal heat radiators 20 and being provided with a cooling chamber 2| from inside which the atmosphere is withdrawn by a conduit 22 cooled by a cooler 23, and returned by a conduit 24, the movements of the atmosphere being obtained by a blower 25. Such an arrangement may be desirable for some metallurgical heating cycles and serves to illustrate that the electric resistance heating effect can be concentrated along the length of the strip at more than one zone by raising the temperature of separate zones by separate heating means. For obvious reasons, the temperature of the strip in the furnace [9 cannot be raised any higher than it can in the furnace 4.

The various rollers shown in the drawing must be electrically isolated by suitable insulation so that the current can travel through the strip throughout its length between the roller-contactors l5 and, for the same reason, the strip extending beyond at least one of the roller-contactor sets must be also electrically isolated. Ordinary electrical engineering principles may be followed in this respect.

I claim:

1. A method of heating a metal charge in a furnace having a plurality of heat radiating means with a limited safe operating temperature maximum, said method comprising heating said means to temperatures close to but below said maximum, directly exposing part of said means to said charge to thereby heat said charge to a temperature close to but below said maximum direct radiation, and passing electric current directly through said charge to raise its temperature above said maximum, while shielding and thermally insulating the rest of said means from direct heat radiation thereto from said charge and while operating said last named means at temperatures close to but below said maximum.

2. A method of heating a continuously movmetal strip in a furnace having a plurality of metal heat radiators with a limited safe operattem perature maximum, said method comprising heating said radiators to temperatures close to but below their safe operating temperature ntiaximum, passing said metal strip through said furnace to directly expose at least one of said radiators thereto sufliciently to heat the strip to a temperature close to but below said maximum, and then to the rest of said radiators while they are shielded and thermally insulated from direct radiation from said strip, said strip being heated above said maximum temperature by passing electric current directly therethrough while juxtaposed to said shielded radiators which are at the same time being operated close to but below said maximum temperature.

3. A continuous metal strip heating furnace including the combination of metal heat radiators positioned along its length, some of said metal heat radiators being directly exposed to said strip, a wall of thermal insulation between certain of said radiators and the strip traveling path, said wall being disposed only throughout a zone near the exit end of the furnace and providing a strip passage, and means for passing electric current directly through the strip at least while it is in said passage.

l. A continuous metal strip heating furnace including the combination of a plurality of heating means positioned along its length, some of said heating means being directly exposed to said strip, a wall of thermal insulation between the rest 01 said heating means and the strip traveling path, said wall being disposed only throughout a zone near the exit end of the furnace and providing a strip passage, and means for passing electric current directly through the strip at least while it is in said passage.

FRED MIESS.

.. REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

